Can I get sendmail to verify an entire e-mail address instead of just the domain?
If the e-mail comes from example.com, you have to look up the MX or A records for example.com to figure out which mail server to ask if the user is valid. The DNS could be slow or unresponsive. The remote server could be slow or unresponsive. If you were to make such checks, a bad guy could bombard you with mail claiming to come from hotmail.com users, and then suddenly you would be connection-bombing Hotmail’s mail servers asking them about the recipients the bad guy is claiming to be. The only safe way to do this is to verify that the sender exists when the sender claims to be in your domain. Naturally you are then in a position to confirm or deny the sender is valid.
